Cancer
Cancer is one of the most common causes of disease and death in the western world. In general, incidence rates increase with age for most forms of cancer. As human populations continue to live longer, due to an increase of the general health status, cancer will affect an increasing number of individuals. The cause of most common cancer types is still at large unknown, although there is an increasing body of knowledge providing a link between environmental factors (diet, tobacco smoke, UV radiation etc) as well as genetic factors (germ line mutations in “cancer genes” such as p53, APC, BRCA1, XP etc) and the risk for development of cancer.
No definition of cancer is entirely satisfactory from a cell biological point of view, despite the fact that cancer is essentially a cellular disease and defined as a transformed cell population with net cell growth and anti-social behavior. Malignant transformation represents the transition to a malignant phenotype based on irreversible genetic alterations. Although this has not been formally proven, malignant transformation is believed to take place in one cell, from which a subsequently developed tumor originates (the “clonality of cancer” dogma). Carcinogenesis is the process by which cancer is generated and is generally accepted to include multiple events that ultimately lead to growth of a malignant tumor. This multi-step process includes several rate-limiting steps, such as addition of mutations and possibly also epigenetic events, leading to formation of cancer following stages of precancerous proliferation. The stepwise changes involve accumulation of errors (mutations) in vital regulatory pathways that determine cell division, asocial behavior and cell death. Each of these changes may provide a selective Darwinian growth advantage compared to surrounding cells, resulting in a net growth of the tumor cell population. It is important to emphasize that a malignant tumor does not only consist of the transformed tumor cells themselves but also surrounding normal cells which act as a supportive stroma. This recruited cancer stroma consists of connective tissue, blood vessels and various other normal cells, e.g. inflammatory cells, which act in concert to supply the transformed tumor cells with signals necessary for continued tumor growth.
The most common forms of cancer arise in somatic cells and are predominantly of epithelial origin, e.g. prostate, breast, colon, urothelial and skin, followed by cancers originating from the hematopoetic lineage, e.g. leukemia and lymphoma, neuroectoderm, e.g. malignant gliomas, and soft tissue tumors, e.g. sarcomas.
Cancer Diagnostics and Prognostics
Microscopic evaluation of a tissue section taken from a tumor remains the golden standard for determining a diagnosis of cancer. For microscopic diagnosis, biopsy material from suspected tumors is collected and examined under the microscope. To obtain a firm diagnosis, the tumor tissue is fixated in formalin, histo-processed and paraffin embedded. From the resulting paraffin block, tissue sections can be produced and stained using both histochemical, i.e. hematoxylin-eosin staining, and immunohistochemical methods. The surgical specimen is then evaluated with pathology techniques, including gross and microscopic analysis. This analysis forms the basis for assigning a specific diagnosis, i.e. classifying the tumor type and grading the degree of malignancy, of a tumor.
Malignant tumors can be categorized into several stages according to classification schemes specific for each cancer type. The most common classification system for solid tumors is the tumor-node-metastasis (TNM) staging system. The T stage describes the local extent of the primary tumor, i.e. how far the tumor has invaded and imposed growth into surrounding normal tissues, whereas the N stage and M stage describe how the tumor has developed into metastasis, with the N stage describing spread of tumor to lymph nodes and the M stage describing growth of tumor in other distant organs. Early stages include: T0-1, N0, M0, representing localized tumors with negative lymph nodes. More advanced stages include: T1-4, N0-4, M0, localized tumors with more widespread growth and T1-4, N1-4, M0, tumors that have metastasized to lymph nodes and T1-4, N1-4, M1, tumors with a metastasis detected in a distant organ. Staging of tumors is often based on several forms of examinations, including surgical, radiological and histopathological analyses. In addition to the staging, there is also a classification system to grade the level of malignancy for most tumor types. The grading systems rely on morphological assessment of a tumor tissue sample and are based on the microscopic features found in a given tumor. These grading systems may be based on the degree of differentiation, proliferation and atypical appearance of the tumor cells. Examples of generally employed grading systems include Gleason grading for prostatic carcinomas and Elston-Ellis grading for breast carcinomas.
Accurate staging and grading is crucial for a correct diagnosis and provides an instrument to predict a prognosis. The diagnostic and prognostic information for a specific tumor subsequently determines an adequate therapeutic strategy for a given cancer patient. The most commonly used method, in addition to histochemical staining of tissue sections, to obtain more information regarding a tumor is immunohistochemical staining (IHC). IHC allows for the detection of protein expression patterns in tissues and cells using specific antibodies. The use of IHC in clinical diagnostics allows for the detection of immunoreactivity in different cell populations, in addition to the information regarding tissue architecture and cellular morphology that is assessed from the histochemically stained tumor tissue section. IHC can be important to support the accurate diagnosis, including staging and grading, of a primary tumor as well as in the diagnostics of metastases of unknown origin. The most commonly used antibodies in clinical practice today include antibodies against cell type “specific” proteins, e.g. PSA (prostate), MelanA (melanocytes), Thyroglobulin (thyroid gland) and antibodies recognizing intermediate filaments (epithelial, mesenchymal, glial) cluster of differentiation (CD) antigens (hematopoetic, sub-classification of lympoid cells) and markers of malignant potential, e.g. Ki67 (proliferation), p53 (commonly mutated tumor suppressor gene) and HER-2 (growth factor receptor).
Aside from IHC, the use of in situ hybridization for detecting gene amplification and gene sequencing for mutation analysis are evolving technologies within cancer diagnostics. In addition, global analysis of transcripts, proteins or metabolites all add important information. However, most of these analyses still represent basic research and have yet to be evaluated and standardized for the use in clinical medicine.
Adenocarcinomas from Colon and Rectum (Colorectal Cancer)
Colorectal cancer, a malignant epithelial tumor that presents as an adenocarcinoma, is one of the most common forms of human cancer worldwide. Data from the GLOBOCAN 2002 database presented by Parkin et al show that around 1 million new cases of colorectal cancer are identified yearly (Parkin D M et al (2005) CA Cancer J Clin 55, 74-108). Further, the incidence of colorectal cancer in the world is approximately 9.4% of all cancers, and colorectal cancer constitutes the second most common cause of death in the western world. The five-year survival rate of colorectal cancer is approximately 60% in the western world but as low as 30% in Eastern Europe and India.
Early detection and surgery with excision of the tumor is currently of critical importance for a successful treatment. For localized tumors, i.e. tumors that have not evolved into a metastasizing disease, surgical intervention with radical resection of the tumor and surrounding bowel and tissues is performed. Colorectal tumors are categorized into several stages according to Dukes' stages A-D or more recently according to the TNM classification. The least malignant tumors (Dukes' stages A and B) are generally associated with a relatively favorable outcome, while highly malignant tumors with metastasis (Dukes' stage C and D) have poor survival rates. Unfortunately, colorectal cancer has often grown to a considerable size before detection and thus metastases are not uncommon. The tumor typically metastasizes to regional lymph nodes, but distant metastasis to the liver and lung are also common.
Symptoms depend on where in the distal gastrointestinal tract the tumor is located, and include bowel distress, diarrhea, constipation, pain and anemia (secondary to bleeding from the tumor into the bowel). Current diagnostics are based on patient history, clinical and endoscopic examination (rectoscopy and colonoscopy), optionally followed by radiological mapping to determine extensiveness of tumor growth. In conjunction with endoscopic examination, tissue biopsies are performed from dubious lesions.
In differential diagnostics, cytokeratin 20 (CK20), an intermediate filament marker abundant in the glandular cells of the GI-tract, is commonly used to diagnose primary tumors in the GI-tract including colorectal cancer. The CK20 marker is not ideal as several other adenocarcinomas also can be positive for CK20 antibodies, whereas not all colorectal cancers are positive. Prognostic information is mainly obtained from tumor staging classification as there are no accepted grading systems or protein markers that provide additional prognostic data. Today there are no available markers that can distinguish tumors of low malignancy grade and low risk for developing into a metastasizing disease from highly malignant tumors with a reduced chance of survival. There is thus a great need for molecular markers that can be used to predict patient outcome and to guide for patient management including therapeutic intervention.